494 THE AMERICAN NATURALIST.  [Vou. XXXIII. 
are, however, brown or black, and those of Potamogeton are 
frequently reddish. The 19 species of the Alismacez all 
have white petals and are attractive to Syrphide. On the 
small flowers of Alisma Plantago-aquatica, Miiller collected 5 
flies of this genus, and on Sagittaria latifolia I have taken 11 
Syrphidze, 6 other Diptera, 2 Coleoptera, and 4 bees. In the 
latter. species the globular mass of yellow stamens, about 40 in 
number, are brought into strong relief by the white petals. 
The primitive color of the Alismaceze was doubtless green, the 
transition of which to white may be illustrated by many exam- 
ples. The involucre of Cornus changes from green to white 
during growth; the floral leaves of Monarda clinopodia are 
white ; the calyx of Polygonum in different species is green, 
greenish-white, and white; the upper sepal of Habenaria obtu- 
sata is green with whitish margins; the perianth of Ornithoga- 
lum umbellatum is white above and green beneath ; Chrosperma 
(Amianthium) is white, but turns greenish with age; I have 
observed the marginal neutral flowers of a culture variety of 
Hydrangea to remain upon the plant for many months and 
change from white to pale green, and many small flowers vary 
from green to white. Of the Vallisneriacez Philotria and 
Vallisneria have white hydrophilous flowers, and Hydrocharis 
is entomophilous. Müller regards Vallisneria as an intermedi- 
ate stage between a water-fertilized and an insect-fertilized 
plant. 
The Graminez with 371, and the Cyperacee with 334 spe- 
cies include nearly two-thirds of northern monocotyledons. 
The same external conditions appear to have been favorable to 
the development and continuance of a great number of forms 
in both families. The flowers are wind fertilized, though they 
are visited occasionally by flies, bees, and beetles for pollen. 
The perianth is nearly suppressed and is represented only by 
scales or bristles. Both the glumes of the Gramineae and the 
bracts of the Cyperacez are often reddish or purplish. This 
coloration is of physiological importance, according to Pick and 
Stahl, and by converting light-rays into heat promotes both in 
young leaves and the organs of flowers transpiration, metabo- 
lism, and growth. Darwin states in “ Variation in Animals and 
